Conventionally, solder used for bonding Surface Mounted Technology (SMT) devices to a printed circuit board (PCB) is deposited on the PCB in the form of a paste. The paste is usually deposited by a screen printing process using a stencil which precisely defines the portions of the PCB to be soldered. Apertures are formed in the stencil at positions corresponding to the required solder pad positions on the PCB. The paste is initially placed on the stencil surface remote from the PCB and forced through the apertures onto the PCB surface by means of a squeegee. The PCB is passed through an oven in which the solder paste melts, wets the pads and the leads of the SMT devices placed on the PCB, and solidifies to bond the devices to the pads. In today's PCB manufacturing environment, a typical stencil will be fabricated from stainless steel foil having a thickness of the order of 0.005 or 0.006 inches. This thickness is governed by the required pitch of the components being used on the PCB, which in this case will be 0.020 and 0.016 inches (referred to as 20 and 16 mil pitch).
Although use of SMT components generally offers high circuit densities, such components generally have significantly less mechanical strength in terms of their physical connection to the PCB than devices mounted by means of through-holes. It is therefore commonly required that components such as cable connectors and chip sockets have high mechanical strength and therefore this type of component is commonly attached by means of plated through-holes.
For plated through-hole devices, it is generally the case that an increased thickness of solder paste is required over and above that possible with standard 0.005 or 0.006 inch thick stencils. This requirement has conventionally been met by employing a separate mini-wave solder process. This however adds complexity and cost to the component attachment process. An alternative process, which avoids the need for an additional wave solder process is described in U.S. Pat. No. 5,129,573.
A further alternative process involves the use of a multilevel stencil, with areas of increased thickness defined where thicker solder paste is required. To date, multi-level stencils have been produced by chemical etching of defined areas of a thicker stencil foil. However, this production method has the disadvantage that the chemically etched portion is uneven in surface topology and thickness thus reducing the quality of the printing process in those areas where fine pitch printing is required.
What is needed therefore is an improved technique for depositing solder paste onto a PCB where varying solder paste thicknesses are required such as for PCBs comprising a combination of SMt and plated through-hole devices.